Crime scene investigators are often challenged with a difficult task of detecting and analyzing forensic evidence that is not visible to the human eye, most notably fingerprints. To detect otherwise invisible forensic evidence, investigators have relied on light in the ultraviolet (“UV”) spectrum, also known as UV radiation. While visible light has a wavelength ranging from about 400 nm to about 750 nm, UV radiation has a shorter wavelength ranging from about 10 nm to about 400 nm. Some types of forensic evidence may include organic substances (e.g., blood, saliva, and semen) that are either readily visible or, upon exposure to UV radiation, may fluoresce and become visible to the human eye. However, other substances like fingerprints have historically required dusting with a fluorescent dusting powder or dyeing prior to UV radiation exposure in order to make the fingerprints visible to the unaided eye. In certain situations, RUVIS technology, which enables a user to see latent fingerprints on nonporous surfaces prior to treatment with powders or dyes, may be used to examine latent fingerprints.
RUVIS technology works on the principle that most nonporous surfaces either reflect or absorb light in the UV spectrum. When UV radiation illuminates a nonporous surface containing a latent fingerprint, the fingerprint stands out as darker or lighter than the background based on whether the surface reflects or absorbs the UV radiation, respectively. Early RUVIS devices were portable, battery operated devices that used an image intensifier with a special photocathode to amplify the reflected UV radiation to allow an investigator to see latent fingerprints. The noisy, grainy, low-resolution, green image on these devices was good enough to detect latent fingerprints, but was inadequate to document and photograph the fingerprints. Attempts were made to capture images of latent fingerprints by including a video or digital camera with the image intensifier. However, these systems were limited by a field of view smaller than two inches by two inches and a relatively low-resolution of the image intensifier. These systems also required collecting and processing of an image before a user could determine whether a suitable latent fingerprint had been captured.
Despite these limitations in RUVIS systems, the forensics community has recently set 1000 pixels per square inch as the minimum resolution to capture latent fingerprints. While modern digital cameras are capable of 12, 14, and 16 megapixel resolutions or even higher, they cannot capture images in the UV spectrum. In fact, to prevent images in the visible spectrum from appearing blurred, modern digital cameras are designed to have spectral sensitivity similar to a human eye, thereby blocking light from the UV spectrum. Glass is used in the lens and window of the sensor for most modern digital cameras, which blocks over ninety percent of the light below 300 nm and most of the light between 300 and 400 nm. Additionally, bandpass filters are often used to block light outside of the visible spectrum, further preventing modern digital cameras from capturing high resolution images in the UV spectrum.